I have not had an opportunity to make it up yet, between leaves and other household activities getting in the way, but from what I can understand it makes it easier to drop the top frame pins/bolts onto the tractor front mount. I know without it the snow blower and the blower mount don't move in unison so it is awkward lifting it on. You lift the blower and the mounting frame drops. I think with the chain when you lift the blower the mount will go up with it allowing it to be easier to drop on the tractor frame.
Not sure I explained it very well, but it is referenced in the operators manual for both attaching and detaching the blower.
So that is theory in practicality does it work well? Maybe someone with more experience can chime in with a better description.